1. Field of the Invention
This invention relates to the molecular gas detection art and more particularly to an improved arrangement for detecting low concentrations of molecular gases by multiple photon absorption techniques.
2. Description of the Prior Art
In many applications there is a need for detecting the presence of molecular gases and identifying the particular molecular gas detected. Such applications include, of course, monitoring of the environmental conditions in many industrial and manufacturing operations, monitoring pollutants such as trace carcinogens or the like and similar applications. It is necessary to detect the presence of and identify various molecular gases. Preferably, such detection and identification may be conducted at the particular location since low concentrations of some molecular gases are some of the components of various mixtures of molecular gases or some of the components sample handling and transport. Additionally, trace contaminents of yet other molecular gases may be introduced during sample preparation and handling.
Optical detection methods are particularly attractive because of high sensitivity, immediate response and suitability for operation at the locations desired and of the optical detection methods ultraviolet spectroscopy has heretofore been widely utilized because it provides comparatively high sensitivity and relative freedom from interference caused by the components of ambient air. However, as applied to molecular gases, the ultraviolet spectroscopy method is hampered since the absorption spectra of complex molecules in the gas phase are broad and generally structure, having many rotational and vibrational bands as well as the presence of hot bands in an overlapping configuration. Fluorescence detection is another method which may be utilized in which excited state energy is emitted by the molecular gas following absorption of energy from an irradiating photon beam. However, spectral interference associated with such fluorescence techniques has limited the lower limit of molecular gas concentration which may be detected by this method.
Photo ionization is one technique which has heretofore been considered for such molecular gas detection and identification. However, single photon photo ionization spectra do not provide a ready identification of the particular gas since the ionization potential of many similar but nevertheless different gases is virtually the same.
Therefore, it has long been desired to provide a detection and identification arrangement for molecular gases which may not only be utilized at the various locations where the gases may be present but also provides the unique identification of the particular gas detected.